Diesel engines are known for certain advantages including low fuel consumption, high torque and low carbon monoxide (CO) and carbon dioxide (CO2) emissions. However, whilst diesel engines tend to produce lower amounts of regulated emissions than gasoline engines, they are also associated with some more difficult to manage emissions, particularly nitrogen oxides (NOx, essentially NO and NO2) and particulate matter (PM). The other regulated pollutant from internal combustion engines is non-combusted or unburnt (including partially burnt) hydrocarbons (HC).
There are primarily two methods of reducing emissions from engines, the first being engine design and management, and the second being aftertreatment of the exhaust gases, although the combination is usually used. Exhaust gas aftertreatment has evolved by research and development of catalytic methods of treating the regulated emissions, and when used with electronic engine management is generally successful in meeting current emission standards. Nonetheless, ever-increasing emission regulations, combined with pressure to reduce fuel consumption and associated CO2 emissions from environmental and global warming perspectives, continue to present challenges to the design of engines and aftertreatment systems.
The first catalytic aftertreatment introduced for vehicular diesel engines was a Diesel Oxidation Catalyst (DOC) comprising a Platinum Group Metal catalyst deposited on a flow-through ceramic or metal honeycomb substrate. Such DOCs are effective to oxidise CO and HC, and are now widely used. Additionally, such DOCs can be effective to reduce the mass of PM by simultaneously oxidising volatile fractions absorbed on a carbonaceous particle.
The use of Selective Catalytic Reduction (SCR) to reduce NOx to innocuous N2 has been used for some 30 years in treating the output of thermal power plants, and is now in widespread use for coal-fired power plants and stationary gas turbine power plants and similar industrial plants with NOx emissions. SCR utilises the addition of a nitrogeneous reductant gas, primarily ammonia or an ammonia precursor such as urea, to the exhaust gas from the plant, and passing the resulting mixture over a catalyst known as an SCR catalyst. SCR systems are under development in USA, Europe and Japan for use in the afterteatment of exhausts from vehicular diesel engines, and are slowly being introduced into the market. Vehicular SCR systems are appreciably more difficult to manage than stationary systems, because of the varying quantity of exhaust gases, varying exhaust gas temperatures and the need to carry a supply of the reductant on the vehicle. Such SCR systems nonetheless appear to offer promise. For completeness, we mention that what may be termed “hydrocarbon SCR”, where a hydrocarbon reductant such as diesel fuel itself, has also been proposed. Such hydrocarbon SCR appears to present rather more problems than the use of nitrogeneous reductants.
Several chemical reactions occur in an NH3 SCR system, representing desirable reactions that reduce NOx to nitrogen. The dominant reaction is represented by reaction (1).4NO+4NH3+O2→4N2+6H2O  (1)
Competing, non-selective reactions with oxygen can produce secondary emissions or may unproductively consume ammonia. One such non-selective reaction is the complete oxidation of ammonia, shown in reaction (2).4NH3+5O2→4NO+6H2O  (2)
Also, side reactions may lead to undesirable products such as N2O, as represented by reaction (3).4NH3+5NO+3O2→4N2O+6H2O  (3)
Commercial SCR catalysts are generally based on a zeolite, particularly a transition metal-modified zeolite (an aluminosilicate) such as Cu- or Fe-modified beta zeolite or Cu- or Fe-ZSM-5. These have a relatively wide temperature activity window. In general, Cu-based zeolite catalysts exhibit better low temperature NOx reduction activity than Fe-based zeolite catalysts.
However, in use, Cu- and Fe-Beta and -ZSM-5 zeolites have a number of drawbacks. They are susceptible to dealumination during high temperature hydrothermal ageing, resulting in a loss of acidity, especially with Cu-based zeolites. Both Beta and ZSM-5-based catalysts are also affected by hydrocarbons which become adsorbed on the catalysts at relatively low temperatures and are oxidised as the temperature of the catalyst system is raised, causing an exotherm which can damage the catalyst structure. This problem is particularly acute in vehicular diesel applications where significant quantities of hydrocarbon can be absorbed on the catalyst on cold-start. Both Beta and ZSM-5 zeolites are also prone to coking by hydrocarbons.
Our WO 2008/132452 describes alternative small pore zeolite catalysts containing at least one transition metal, such as silicoaluminophosphate. Hereinafter, however, “molecular sieve”-based catalysts will be used in the description of the present invention to include such catalysts as the silicoaluminophosphates (SAPO) having a zeolite-like structure. Certain authorities consider that the term “zeolite” should only be used for a silicoaluminate. We note that the SAPO-based catalysts of WO 2008/132452 are exemplified in a system comprising an upstream oxidation catalyst, an intermediate PM filter and a final SCR catalyst having an ammonia slip catalyst.
The removal of PM from diesel exhaust gases is generally realised by some form of filter or partial filter. A large number of filter designs have been proposed in the patent literature. Currently, the state of the art filter is a ceramic or ceramic-like wall flow filter, carrying a PM combustion catalyst, known as a catalysed soot filter (CSF). A number of variations on CSFs have been proposed, including the coating of the filter with a NOx absorber catalyst (NAC) or a SCR catalyst. As an example, we refer to, and incorporate herein the entire teaching of, WO 2005/01647. In this patent application, it is proposed to coat a filter substrate with an SCR catalyst composition. An essential feature of WO 2005/016497 is the positioning of a DOC upstream of the injection point for the SCR reductant, and hence upstream of the combined SCR and CSF.
JP Published Application 03130522 (Mitsubishi Heavy Industry) proposes a stationary diesel engine having a turbocharger, which is provided with an ammonia injector and an SCR catalyst fitted between the engine and the turbocharger. It is not believed that this design has been successfully introduced in the 20 years since it was proposed.